U.S. Pat. No. 3,280,183, granted Oct. 18, 1966 to Maggiolo, discloses a method of producing a, dicarboxylic acids from eight to twelve carbon atom cycloalkenes. This method begins with a cycloalkene having the same number of carbon atoms as the dicarboxylic acid. The method is said to allow these acids to be produced economically.
There are two steps to the method. First, the cycloalkene is dissolved in a solvent at a concentration xe2x80x9cat least aboutxe2x80x9d 10% (the solubility depends on the particular cycloalkene and solvent; a solubility of 20% is disclosed for cyclooctene in isobutyric acid). The solvent is a monobasic acid such as acetic acid, propionic acid, isobutyric acid, etc. The resulting mixture is contacted with a gas containing 2 to 7% ozone at a temperature of 25xc2x0 C. or less for up to several hours until the cycloalkene is totally reacted to ozonide (ozonolysis). Lower temperature ozonolysis is preferred for economic reasons. When the ozonolysis is complete, the temperature is raised to the range of 65 to 110xc2x0 C., and the mixture is contacted with an oxygen-containing gas (oxidation). After a period of 1.5 to 13 hours under these conditions, the reaction is complete. Following the oxidation, the product, xcex1,xcfx89 dicarboxylic acid, is purified by vacuum distillation.
Both the temperature of ozonolysis and the conditions for oxidation have a direct effect on the ultimate yield in the conversion of the cycloalkene into xcex1, xcfx89 dicarboxylic acid. Other prior art in this field supports the notion that ozonolysis is most selective at lower temperatures. All of the examples in the Maggiolo patent involve batch reactions in which the problems of continuous processes are not encountered. One of the more significant problems is that of limited solubility of cycloalkenes in organic solvents. If, in an ozonolysis reactor, cycloalkene were added to solvent in excess of the solubility limit, the cycloalkene/solvent mixture would separate into two liquid phases: one consisting of pure cycloalkene, and the other comprising cycloalkene, solvent and ozonide. The separation would result in a reduction in ozonide yield due to the formation of unwanted byproducts in the pure cycloalkene phase. If, however, the cycloalkene were kept at the solubility limit, the molar concentration of ozonide would be expected to be the same as that of the cycloalkene.
It would be desirable to have a continuous process for converting cycloalkenes into their corresponding xcex1, xcfx89 dicarboxylic acids. In such a continuous process, the solvent and cycloalkene would be fed continuously to an ozonolysis reactor. Since the heat of reaction between ozone and cycloalkene is known to be very high, on the order of 100 kcal/gmole, a large amount of heat would have to be conducted away from the reactor to maintain an appropriate reaction temperature. Finally, as is known for continuous processes in general, the higher the loading of reactants, the higher the efficiency and yield.
The present invention is directed specifically to the ozonolysis step of a continuous process for converting a cycyloalkene into its corresponding ozonide. Ultimately, the ozonide may be oxidized to obtain an xcex1, xcfx89 dicarboxylic acid, as in U.S. Pat. No. 3,280,183. Alternatively, the ozonide may be efficiently rearranged to make a product having both an aldehyde end group and an acid end group (aldehyde/acid). The aldehyde/acid can then be used, for example, as an intermediate for making cyclic lactones, cyclic lactams and a wide variety of other useful compounds.
Specifically, in its first aspect the present invention is a process for producing the ozonide of a C6 to C12 cycloalkene comprising:
contacting in a reaction vessel (1) a mixture of the cycloalkene and a solvent therefor with (2) a carrier gas containing about 1 to 2% ozone at a temperature of about 20 degrees,
withdrawing from the reaction vessel a product stream comprising the carrier gas and a liquid comprising the ozonide dissolved in the solvent,
separating the liquid from the gas,
withdrawing heat from the liquid to form a cooled product stream,
mixing a portion of the cooled product stream with additional cycloalkene to form a single, liquid phase cycloalkene-supplemented recycle stream, and
adding the recycle stream to the reaction vessel.
In its second aspect the present invention is a process for producing the ozonide of a C6 to C12 cycloalkene comprising:
contacting in a cooled multistage distillation column (1) a mixture of the cylcoalkene and a solvent therefor with (2) a countercurrently flowing carrier gas containing about 1 to 2% ozone at a temperature of about 20 degrees C.;
withdrawing carrier gas from the reaction vessel;
withdrawing from the reaction vessel a liquid comprising the ozonide dissolved in the solvent;
returning to the reaction vessel a portion of the liquid while adding additional cycloalkene to the reaction vessel; and
recovering a portion of the liquid as a product.